egm96geoid (Mapping Toolbox)

Read 15-minute gridded geoid heights from the EGM96 geoid model of the earth

Syntax

[map,maplegend] = egm96geoid(scalefactor)
[map,maplegend] = egm96geoid(scalefactor,latlim,lonlim)

Background

Although the earth is round, it is not exactly a sphere. The shape of the Earth is usually defined by the geoid, which is defined as a gravitational equipotential surface, but may conceptualized as the shape the ocean surface would take in the absence of waves, weather and land. For cartographic purposes it is generally sufficient to treat the earth as a sphere or ellipsoid of revolution. For other applications, a more detailed model of the geoid such as EGM 96 may be required. EGM 96 is a spherical harmonic model of the geoid complete to degree and order 360. This function reads from a file of gridded geoid heights derived from the EGM 96 harmonic coefficients.

Description

[map,maplegend] = egm96geoid(scalefactor) reads the data for the entire world, downsampling the data by the scale factor. The result is returned as a regular matrix map and associated map legend. Heights are given in meters in the tide-free system.

[map,maplegend] = egm96geoid(scalefactor,latlim,lonlim) reads the data for the part of the world within the latitude and longitude limits. The limits must be two-element vectors in units of degrees. Longitude limits can be defined in the range [-180 180] or [0 360]. For example, lonlim = [170 190] returns data centered on the dateline, while lonlim = [-10 10] returns data centered on the prime meridian.

Examples

Read the EGM 96 geoid grid for the world, taking every 10th point.

```
[map,maplegend] = egm96geoid(10);
```

Read the a subset of the geoid grid at full resolution and interpolate to find the geoid height at a point between grid points.

[map,maplegend] = egm96geoid(1,[-10 -12],[129 132]);
z = ltln2val(map,maplegend,-11.1,130.22,'bicubic')
z =
    53.4809

Remarks

This function reads the 15-minute EGM96 grid file "WW15MGH.GRD". The grid is available from

<http://www.nima.mil/GandG/wgs-84/egm96.html>

as either a DOS self-extracting compressed file or a UNIX compressed file. Do not modify the file once it has been extracted.

Information on the dataset may be found at:

<http://cddisa.gsfc.nasa.gov/926/egm96/egm96.html>.

Maps will extend a half a cell outside the requested map limits.

There are 721 rows and 1441 columns of values in the grid at full resolution. The lower resolution atlas data in GEOID.MAT is derived from the EGM 96 grid.

See Also

ltln2val
Returns map code value associated with positions

`ltln2val`	Returns map code value associated with positions

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